LONDON — Leigh Osborne and his partner, Graham Voce, spotted the derelict water tower in 2010, from their apartment on the 36th floor of the Strata Tower in the city’s Elephant and Castle neighborhood. From what they could tell, the lonely brick water tower in the nearby Kennington area, surrounded by a ring of new condominiums, was vacant and disintegrating. But Osborne, a property developer who is now 40, had to have it.

By the time the landmarked water tower was listed for sale later that year for 380,000 pounds (now about $603,000), he had primed Voce, an executive secretary at the International Institute for Conservation of Historic and Artistic Works, who is now 50, for a big change.

Soon, they had sold their high-rise apartment and plunged into a restoration project that would prove to be something of a money pit.

“I thought we would invest 600,000 pounds” — or about $951,000 — “and I was completely wrong,” Osborne said.

By the time the work on the 4,000-square-foot space was completed last fall, they had spent more than 2 million pounds, or $3.2 million.

In England, landmark buildings are thorny projects to take on, as Osborne discovered. This particular eight-story tower, the tallest building in London when it was completed in 1867, brought clean water to the Lambeth Workhouse and Hospital, which was demolished in 2007 to make room for housing. The tower’s Venetian Gothic styling and imposing height recall the Venice Campanile in St. Mark’s Square, incorporating features like diagonal brick buttresses with pointed headstones, stone lintels and giant arcaded framing.

When it was no longer needed in the 20th century, however, the tower was abandoned. It became a roost for generations of pigeons whose droppings bore the seeds of plants and trees that took root in the detailed brickwork, causing fantastically expensive damage that Osborne was legally required to fix.

There was also the complicated matter of turning a water tower into a home, which required shedding several feet of internal brick walls to carve out decent-size living spaces. The couple built a modern extension as well, to house a large kitchen, living room, gym and reception area. Digging the foundations for the addition, construction workers uncovered Georgian-era roads and houses, which added weeks of costly excavations and construction to the already soaring bill.

Nevertheless, Osborne had established an ambitious eight-month deadline, and he was determined to keep it, setting the scene for a frenetic building blitz that began last winter.

“It was incredibly complicated, because we had a sharp deadline and an evaporating budget,” he said. “We just threw money at this thing to employ the dozens of workers that were needed to make it happen.”

Inside the tower, the construction crew power-washed more than a century of encrusted filth from the brick walls. The original twisting cement staircase now links up with a new elevator that climbs eight stories and feeds into three bedrooms and four baths. Osborne calls the living room on the top floor the Prospect Room. In its former life, it was a square cast-iron tank designed to hold 750,000 gallons of water. Osborne retrofitted the space with expansive windows on all sides, creating 360-degree views of the city. It’s a little like standing in a pod on the London Eye.

The 980-square-foot modern addition, which Osborne refers to as the Cube, is three floors of glassed-in space that provides the couple with the kind of large, airy rooms — an open-plan kitchen, a modern living area — that they don’t have in the water tower.

In the end, the result was surely worth the onerous effort and expense.

Even so, Osborne allows: “As I sit here today, I can tell you that to finish and restore this house, I sold properties, remortgaged a handful of homes, put 98,000 pounds” — or $155,000 — “on my credit cards and borrowed money from friends. All the things you should never do.”